1. Field of the Invention
The present invention relates to an improvement on a pulsated combustion apparatus and a method for controlling such a pulsated combustion apparatus, the apparatus and method being suitable for a ceramic article-firing furnace, and the like.
2. Related Art Statement
Burners for ceramic article-firing furnaces or the like generally have been controlled by adjusting the degree of opening of each burner during firing within a given narrow width, so that a target firing temperature curve may be realized. However, throttling the degree of opening of the burner generally decreases the amount of combustible gas from the burner. This controlling method has the problem that the kinetic energy of the combustible gas is reduced and can become insufficient depending upon the temperature range. Reduced kinetic energy of combustion gas makes the temperature distribution inside the furnace poorer and as a result causes an increase in the percentage of defective articles fired in such furnace. In order to solve such shortcomings, a combustion-controlled method called "pulsated combustion method" is known.
In the pulsated combustion method each burner in each zone inside the ceramic-firing furnace or the like is controlled by intermittently turning on and off the burner according to selected ignition timing at short time intervals under a given cycle. When the burner is turned on, the combustion state is kept at a maximum. On the other hand, when the burner is in a turned off state, the combustible state of the burner is kept low or even extinguished. The on-off cycle, the ignition timing and the amount of the combustible gas are set so that a desired firing curve and a maximum turn-down (maximum difference in output of the burner between the turn-off and turn-on) may be obtained.
Since the pulsated combustion method feeds a large amount of fuel and air into the furnace during a rising time of the turned-on operation, a large amount of kinetic energy is created, averting the problem of the kinetic energy becoming insufficient during the firing. Thus, a prior art shortcoming that the percentage of defective articles increases due to the poor temperature distribution inside the furnace is often eliminated. As a burner-ignition system, direct ignition using a high temperature heat source such as a spark plug or electric heating wire may be used. The pulsated combustion system includes a high-low system and a high-off system. "High" means the maximum combustible gas output state when the burner is turned on, "low" means a pilot burning state in which pilot burning flame is always maintained even when the combustion is in the turned-off state, and "off" means that the burner is completely extinguished when the combustion is turn-off state. The high-low system includes a direct ignition system and a pilot burner system. In the direct ignition system, the flame is maintained in the "low" state by using a nozzle of the same burner used in the "high" state. In the pilot burner system, the flame in the "low" state is maintained by using a pilot burner different from the nozzle of the burner to be used in the "high" state. The present invention relates to the former in the high-low system, that is, the invention relates to the technique (direct ignition technique) in which the flame in the "low" state is maintained by using the nozzle of the same burner used in the "high" state.
In the case of the direct ignition system, the flame of the "low" state is first formed by a spark plug in the operating furnace. Thereafter, pulsated combustion is effected by opening and closing an air feed valve and a fuel gas feed valve. In this system, while the same burner, the same air source, and the same fuel source are employed in both the "low" state and the "high" state, the feed lines are partially changed. At a starting time of each cycle of the pulsated combustion, the air feed valve and the fuel feed valve are opened to feed a mixed gas of combustion air and the fuel gas to the burner, so that the freshly fed mixed gas begins to be burned by using the flame in the "low" combustion state as an ignition source. At the termination time of each cycle of the pulsated combustion, the two valves are closed to convert the burner to the "low" combustion state.
In contrast, the pilot burner system ignites the pilot burner first to form the flame in the "low" state. At starting time, each cycle of the pulsated combustion, the fuel gas and combustion air are fed into the furnace through respective "high" combustion lines, and combustion in the `high" state is started by using the flame of the pilot burner as an ignition source. At termination time each cycle of the pulsated combustion, the fuel gas feed line and the air feed line for the "high" combustion state are shut, and combustion is maintained only through the pilot burner.
FIG. 1 shows a pulsated combustion apparatus of a conventional direct ignition type in the pulsated combustion system for feeding combustion air and the fuel as to the burner in a pulsated manner. In FIG. 1, a burner 1 attached to a wall portion of a ceramic article-firing furnace is connected with a combustion air feed line 2 and a fuel gas feed line 3. The air feed line 2 is provided with a valve 4 for adjusting a feed amount of air, whereas the fuel gas feed line 3 is provided with a valve 6 for adjusting a feed amount of the fuel gas.
The valve 4 is provided with an actuator 5 for controlling the degree of opening of the valve 4, and a control unit and an air source are connected with the actuator 5. The valve 4 and the valve 6 are interlockingly connected with each other by means of a mechanical link 7. The air feed line 2 and the fuel gas feed line 3 are provided with an air bypass line 8 and a fuel bypass line 9 for bypassing the valves 4 and 6, respectively. The air bypass line 8 and the fuel bypass line 9 are provided with flow rate control valves 10 and 11, respectively. As shown, an ignition plug 12 is provided near an opening of a burner 1.
In the ceramic article-firing furnace or the like, its interior is divided into a plurality of zones, and one or more of the above pulsated combustion apparatuses are provided for each of the divided zones so that combustion of the pulsated combustion apparatuses may be controlled to give a desired temperature distribution in each of the zones.
In the pulsated combustion apparatus of FIG. 1, the fuel gas mixture is first ignited by the ignition plug 12; at turned-off time, the burner is kept at a low combustion level in the state that air and the fuel gas are fed to the burner through the bypass lines 8 and 9, respectively. During a transition period to the turn-on state, a turn-on signal is fed to the actuator 5 from the control unit, and then the valve 4 is opened to a given degree of opening as shown in FIG. 2. On the other hand, the opening motion of the valve 4 of the air feed line 2 is transmitted to the valve 6 of the fuel gas feed line 3 through the link 7, so that the valve 6 begins to be opened at a given time lag .DELTA.t, and then opened to a given opening degree. After the valve 6 begins to be opened, the air-fuel gas mixture is burned by a pilot combustion flame. By the present motion of the link, the feeding of air preferentially begins to be stopped at the time of terminating each combustion cycle, whereas the valve 6 begins to be closed after the air valve 4 begins to be closed at a given time lag .DELTA.t' via link 7 to stop the feeding of the fuel, as shown in FIG. 2.
In order to control the valves in a reverse manner to that of FIG. 2, the link may be adjusted so that at starting time of each combustion cycle, the fuel gas valve 6 is first opened and thereafter the air valve 4 is opened, whereas at termination time of the combustion cycle, the fuel gas valve 6 is first closed, and thereafter the air valve 4 is closed. This controlled state is shown in FIG. 3.
In FIG. 2, the feed pressure of combustion air is inevitably increased at the time of starting the combustion in each cycle preferentially to increase the feed pressure of the fuel gas, so that the combustible mixed gas becomes too lean. On the other hand, in FIG. 3, at the time of terminating the combustion of each cycle, the feed pressure of the combustion air is inevitably decreased subsequent to a decrease in the fuel gas, so that the combustible mixed gas becomes too lean. Therefore, in the conventional pulsated combustion apparatus, there is the possibility that the pilot combustion flame is extinguished at the time of starting or terminating of the combustion in each cycle. Further, during the cycle of turning on and off the combustion air and the fuel gas, the ignition timing and the amount of the fuel gas are set in view of various factors to prevent the pilot flame from being extinguished. It is not easy to effect such setting because feeding and stopping of air and the fuel gas are controlled in the state that the valves 4 and 6 are interlocked.